Photolithography is a process commonly used in semiconductor fabrication for selectively removing portions of a thin film from or depositing portions of a film onto discrete areas of a surface of a semiconductor wafer. A typical photolithography process includes spin coating a layer of a light-sensitive material (commonly referred to as a “photoresist”) onto the surface of the semiconductor wafer. The semiconductor wafer is then exposed to a pattern of light that chemically modifies a portion of the photoresist incident to the light. The process further includes removing one of the incident portion or the non-incident portion from the surface of the semiconductor wafer with a chemical solution (e.g., a “developer”) to form a pattern of openings in the photoresist on the wafer. Subsequently, portions of the thin film on the surface of the semiconductor wafer can be selectively removed from or deposited onto the surface of the wafer through the openings of the photoresist mask. The photolithography process can be repeated to form layers of microelectronic features on or in the semiconductor wafer.
In the foregoing process, new patterns are aligned with existing patterns of features already on the semiconductor wafer. If a new pattern is not aligned accurately, the new pattern may overlap and/or otherwise interfere with existing patterns on the semiconductor wafer, which may render the newly-formed features inoperable. To ensure proper alignment, conventional processing methods utilize multiple fiducials on the semiconductor wafer at various stages throughout processing. As used herein, a “fiducial” refers to a set of fiducial marks that can be identified by a photolithography tool. For example, photolithography tools use fiducials as reference points or patterns for alignment with respect to the semiconductor wafer before exposing the surface of a wafer to a pattern of light.
FIGS. 1A-1F are partially schematic cross-sectional views illustrating a portion of a workpiece 100 in a prior art method for aligning a UBM structure to a TSV utilizing at least two fiducials. As shown in FIG. 1A, the workpiece 100 includes a semiconductor device 101 (only a portion shown) removably attached to a temporary carrier 114 via an adhesive material 112. The semiconductor device 101 includes a redistribution structure 110, a dielectric material 108 on the redistribution structure 110, and a substrate 102 on the dielectric material 108. The semiconductor device 101 further includes a plurality of TSVs 104 (only one shown) extending from the redistribution structure 110 through the dielectric material 108 and the substrate 102 to a backside 100a of the device 101. FIG. 1A shows the semiconductor device 101 after passivation materials 116 and 118 have been deposited on the backside 100a surface of the device 201 and subsequently planarized to expose a backside surface 103 of the TSV 104.
As shown in FIG. 1A, the workpiece 100 includes a first fiducial 111 (shown schematically) positioned at a frontside of the device 101 between the carrier 114 and the redistribution structure 110. The first fiducial 111 is used during various processing steps to align a photolithography tool to the backside 100a of the device 101. Using infrared illumination, a photolithography tool positioned at the backside 100a of the device 101 detects the first fiducial 111 through the substrate 102, dielectric material 108, and redistribution structure 110. The first fiducial 111 provides a reference point to align the photolithography tool to the workpiece 100. Certain materials subsequently deposited on the backside of the device 101 are opaque, however, which limits the photolithography tool and prevents proper alignment with the device 101 for subsequent processing steps.
Conventional methods address this problem by including a second fiducial 122 at the backside of the device 101, as shown in FIGS. 1B and 1C. Referring first to FIG. 1B, a transparent photo-sensitive polymer material 120 is deposited on a backside surface of the device 101. Using IR illumination, the photolithography tool 170 identifies the first fiducial 111 to align the pattern for the second fiducial 122. As shown in FIG. 1C, the polymer material 120 is patterned and developed to form the second fiducial 122 at the backside surface of the device 101. An opaque seed layer 124 made from copper or another seed or barrier material can then be formed on the second fiducial 122 and the backside of the device 101.
As shown in FIG. 1E, the photolithography tool 170 then aligns the pattern for a UBM structure based on the protuberance in the seed layer 124 created by the second fiducial 122 to form an opening 132 through a resist material 130 on the backside of the device 101. As shown in FIG. 1F, a UBM structure 140 is then formed in the opening 132 and remaining resist material 130 is removed.
The above-described method can be lengthy and expensive, which is exacerbated by the deposition and patterning of the second fiducial. Accordingly, several improvements for reducing the time and cost of these devices and methods would be desirable.